Current limiter



R. R. GOODRICH CURRENT LIHITER Filed April 1. 1942 3nnenloi Robert H.Goodrtch (Itto rneg to a conventional thermionic cathode.

Patented Sept. 4, 1945 UNITED STATES PATENT OFFICE- M22323... Robert R.Goodrich, Merchantvilie, N. 1., assignor to Radi of Delaware Corporationof America, a corporation Application April 1, 1942, Serial No. 437,168

6 Claims.

This invention relates to the regulation of electrical current andparticularly to improved current regulating means for electron dischargedevices of the type normally employing operating potentials of severalthousand volts.

While the invention will be disclosed as embodied in a currentregulating system for a socalled point microscope," it will be apparentthat there are many other applications for the invention and that thedisclosure in this respect is merely illustrative for purposes ofexplaining the inventive concept. It is now appreciated by those skilledin the art of electron discharge tubes that, for certain applications, a"point cathode is to be preferred By way of example in so-called beamtubes wherein a beam of minimum cross-sectional area is desired, the useof a point cathode may reduce or obviate the necessity for employingauxiliary magnetic or electrostatic means for compressing the beam. (Asto this see, by way of example, Ramberg et al. U. 8. Patent 2,271,990).

The principal objection to the use of a. point cathode is the tendencyof the point to burn out the presence of residual gas in the tube.Attempts to obviate this difliculty, as by the use of a protectivesystem of the type employing a resistor in series with the load andwhich depends for its operation upon an increase in current through saidresistor, have not since such systems are based upon the feedbacprinciple and possess an inherent time lag which, however small,prevents the limiting action from taking place prior to the passage ofthe current which the said system is designed to control.

At first glance it might appear that the foregoing objections toconventional "feedback systems for regulating the current supplied to apoint cathode might be overcome by the use in a "feed forward" systememploying a conventional tube (say an RCA pentode type 6J7) possessing asubstantially flat plate-current characteristic. objection to the use ofconventional tubes in such a regulating system resides not only in thefact that they are usually incapable of standing up under the voltageswhich they would be obliged to sustain but in the fact that theirinterelectrode capacitance is so great as to provide a capaci ive pathto the load for at least a surge of the current whose passage it isdesired to preventbeen successfultrons from the The- 'vention is toAccordingly, the invention is to provide an improved current regulatingsystem for high voltage discharge tubes, and one which obviates theforegoing and other less apparent disadvantages to known regulatingsystems.

Another and related object of the present inprovide an improved currentregulating discharge tube having a low internal capacitance, a compactstructure, and one nevertheless capable of use in connection with a loaddevice which is normally subject to an operating potential of severalthousand volts.

The foregoing and other-objects are achieved, in accordance with theinvention, by the provision, in a "feed forward" regulating system, of abeam tube of special design and wherein the electron current whichimpinges a metal target or second anode is independent of variationsinvoltage incident to changes in the resistance of the utilizationcircuit. 7

Certain preferred details of construction together with other objectsand advantages will be apparent and the invention itself will be bestunderstood by reference to the following specification and accompanyingdrawing wherein Figure 1 comprises a longitudinal sectional view of acurrent limiting beam tube constructed in accordance with the invention;

Figure 2 is a sectional view taken on the line 2-2 of Fig. 1, and

Figure 3 comprises a diagrammatic view partly in section showing thedevice of Fig. 1 connected for use, as a current limiter, with aso-called "point microscope."

In the drawing wherein like reference characters designate the same orsimilar parts in all figures, I designates a highly evacuated tubularenvelope containing an electron gun 3 adjacent one end and, adjacent theother end, a second anode or target electrode I upon which the elecgunimpinge. The target electrode I comprises a-fiat metal plate whichextends in a plane normal to the long'axis of the gun and is shownsupported upon a press 1 by means of a connector 0 which extends throughthe press to the exterior of the envelope. The plate or target 5 isprovided with rounded edges la to prevent cold emission and, in order tomore effectively dissipate heat, comprises a surface of an areaconsiderably greater than required for the reception of the beam ofelectrons from the gun I. A plate ll of similar diameter and contour ismounted on the leading end of the gun in principal object of the present2 I register with the target plate I to ensure a symtrons in theirjourney to the target. The plate i I is providedwith a central apertureI in through II in the. instant case comprises a helicallywound fiatpancake filament which is supported, as on conductive leads Ila, lib.within the cylindrical control electrode II, and has its emissivesurface presented to a central aperture I511 in the said cylinder. Agrid wire lib stretched ass-soar \i metrical held in the space traversedby the elec- I reduced. Since the limiter tube l is in series with thepoint cathode I} of the tube the voltage' across the limiter increasesby the same amount that the voltage across the tube 25 drops, thus theoverall voltage remains the same. In the absence of secondary emissioneii'ects in the limiter tube the large increase in voltagethereacross'has no eil'ect upon the electron current which impinges thesecond anode I since the tubular'conduit l1 eflectively shields theelectron source ll from the intensified electrostatic held between theend plate H and the second anode plate 5. Further, since the spacingbetween the across the aperture Na and conductively secured to thecylinder it permits the electron current to be controlled withoutaltering the diameter 'of the beam. This provides a sharp control and isespecially desirable in the instant, case since it prevents spreading ofthe beam and inhibits the generation of secondary or impact electronswhich would result if the primary electrons struck the inner surface orthe first anode II or the inner edge of the end plate H. The first anode[1- is provided with a pair of beam forming diaphragms l9 and 2|, havingcentral apertures which are aligned with the aperture Ila in the 'endplate II.

variations, due to arcing, in the resistance 0! the said load: Diameterof grid and first anode cylinders 1%". Length of grid cylinder A";length of anode cylinder 1''. Diameter of grid aperture 0.15"; diameterof grid wire 0.007". Grid-anode space 0.040". Spacing between end plateII and second anode 8 1". Spacing of focusing diaphragms l9, 2! 0.5".Diameter of beam forming apertures 0.125". Diameter of aperture in endplate I i 0.25". The parts were made of tantalum carefully outgassed andtreated with about 100 kilovolts to burn oi! all cold emission points.

Referring to Fig. 3, the electron current which impinges the second anod5 of the regulator tube i is applied as by means of a lead 5b to aneedle like point-cathode 23 of an electron discharge device 25 which,in the instant case, comprises an electron-microscope similar to the oneshown in Ramberg et a1. U. S. Patent 2,271,990. As disclosed in the saidpatent, one or more auxiliary electrodes 21 and 29, when suppliedthrough leads 21a and 29a, respectively, with potentials severalthousand (say, 5000) volts positive with respect to the cathode 23,serve to establish and maintain a "cold discharge" of electrons from thepoint 23a- Though it is usual. to outgas such a tube just prior to use,it may happen that during its operation'enough occluded gas willbedriven out of the metal or glass to render the spacev betw en egg +3 23aof the cathode 28 and i-tsnext adiacent eiectrcilc it at stalls-ins resithat the vintage area therebetween is m energizec],

plate H and 5 is so great that the capacitance therebetween isnegligible there can be no capaci- 'tive discharge of currenttherebetween (as there would be in the case of a conventional pluralgrid tube) which can .be transmitted to the load device 25.

I claim as my invention:

1. A system consisting of -a current limiter comprising an evacuatedenvelope containing an electron emissive cathode and an anode, and autilization circuit connected to said anode, said utilization circuitincluding a path of variable resistance, and electrode means ofnegligible capacitance between said cathode and anode for maintaining aconstant flow of electrons therebetween substantially independent ofvariations in the resistance of said utilization circuit.

2. The invention as set forth in claim 1 and wherein said electrodemeans. comprises a conduit through which said electrons pass in theirjourney to said anode 3. The invention as set forth in claim 1 andwherein said utilization circuit comprises a point cathode and adischarge maintaining electrode mounted in spaced relation with respectto said point cathode.

4. The invention as set forth in claim 1 and wherein the spacing betweensaid anode and the electrode next adjacent thereto is so great as toprevent a capacitive discharge of current therebetween when theresistance of said utilization circuit is varied. I

5. A current limiter comprising an electron emissive cathode, a tubularelectrode and a platelike metal anode upon which electrons from saidcathode impinge mounted in the order named in spaced relation along acommon axis within an evacuated envelope, and leads individual to saidtubular electrode and said anode for establishing a difference ofpotential therebetween, the

spacing between said anode and the adlacent end of said tubularelectrode being so great as to prevent a capacitive discharge oi currenttherebetween when said device is subjected to extreme variations in thepotential applied to said'electrodes.

6. The invention as set forth in claim 5 and wherein said tubularelectrode terminates in a plate-like surface of a diameter correspondingsubstantially to that of said anode. whereby a b tia y u i e e rostaticseats snap shill ehetseen when said electrodes are seems a. seam es-

